Assessing Lattice Thermal Conductivity of Topological Insulator Bi₂Se₃ by Temperature-Dependent Raman Spectra

Topological insulators (TIs) have emerged as a fascinating material class that exhibits unique electronic properties, making them promising candidates for various technological applications. One of the promising areas where TIs hold great potential is thermoelectricity. Unlike conventional materials, TIs possess a non-trivial band gap in their bulk states but harbor conducting surface states due to the presence of robust, spin-polarized, and topologically protected electronic states. The conducting surface states of TIs, known as topological surface states (TSS), have a linear energy-momentum dispersion relationship, which can lead to high electron mobility and improved electrical conductance. This unique electronic structure offers an advantage over conventional thermoelectric materials, where achieving both high electrical conductivity and low thermal conductivity is challenging.<br/>The pursuit of highly efficient thermoelectric (TE) materials hinges on understanding their electrical conductivity (σ), Seebeck coefficient (S), and thermal conductivity (k_L). Among these, thermal conductivity (κ_L) plays a pivotal role and is a critical factor in TE efficiency. This study delves into the reduction of κ_L, focusing on hexagon-shaped nanocrystals cluster of Bi₂Se₃, synthesized via the hot-injection technique employing non-toxic solvents. The nanocrystals' temperature-dependent Raman spectra were analyzed to determine the average Debye temperature (θ_D) and Gruneisen parameter (γ), utilizing the bond order–length–strength correlation theory(BOLS).<br/>At room temperature, κ_L of the Bi₂Se₃ nanocrystals was evaluated as 1.85 Wm^-1K^-1 using θ_D and γ optained from BLOS theory, closer to the simulated values of 1.4 Wm^-1K^-1 in-plane and 0.4 Wm^-1K^-1 out-of-plane direction.<br/>Our theoretical studies adopting 3 phonon process demonstrate that over 54 % of in-plane cumulative κ_L, and around 12 % of out-of-plane cumulative κ_L are contributed by the modes below 2 THz, which are dominated by the Bi atom. The ratio of in-<br/>plane cumulative κ_L to out-of-plane cumulative κ_L at 5 THz is ~ 3.58, establishing the anisotropic κ_Lof Bi₂Se₃. Further our mean free path (λ) dependent κ_L studies demonstrated the cumulative κ_L in the plane is dominated by the phonons with a mean free path below(λ)~ 15 nm, while along the out-of-plane directions, the phonons with an λ of ~ 5 nm have strong contributions. The Bi₂Se₃is an anisotropic system, and the cumulative thermal conductivity as a function of the λ displays that a major contribution is arising from the shorter mean free path. This scattering is the main cause of low lattice thermal conductivity in the investigated system.<br/>The introduction of nanostructuring-induced grain boundaries in Bi₂Se₃ obstructs long-mean-free-path phonons. Furthermore, intentional doping can also significantly reduce the phonon mean free path, effectively lowering κ_L. Anisotropic phonon scattering, facilitated by weak Van der Waals forces between quintuple layers in the out-of-plane direction, alongside acoustic-optical phonon scattering and anharmonicity, impeded efficient thermal energy transport in Bi₂Se₃, resulting in reduced k_L.<br/>This research demonstrates a method for determining κ_L from temperature-dependent Raman spectra and sheds light on the potential of intentional nanostructuring and doping to optimize TE materials. These findings are instrumental in advancing the understanding of materials that enhance the TE quality factor and, consequently, the figure of merit, offering a promising avenue for future research in the realm of thermoelectrics.